Bellcrank Mechanisms For Stirling E,Ngines.pdf

BELLCRANK MECHANISMS FOR STIRLING E,NGINES

James R. Senft

University of Wisconsin - R.F.

River Falls, WI 54022

Victor J. Senft

University of Minnesota

Minneapolis, MN 55455

ABSTRACT

This paper describes a family of linkage drive systems for

Stirling engines containing several new members. These

mechanisms are adaptable to all three configurations of Stirling

engine, impose minimal side loads on pistons and displacer rods,

and include compact forms suitable for pressurized high

performance engines. This group of drive systems is generated

by a simple common scheme. Near sinusoidal motion is taken

from a crankshaft carrying a single crankpin by two connecting

rods each driving a bellcrank. The stationary pivots of the

bellcranks are located so that their oscillatory motion has the

phase angle separation required between the piston and displacer.

The bellcranks are further configured to bring the third pin

motion to a location suitable for coupling with the piston or

displacer of the engine in a way which minimizes side loading.

The paper presents a number of new linkage drives from the

dual bellcrank family and indicates how they are embodied in

beta and alpha type Stirling engines. The paper includes a design

for a small multipurpose engine incorporating one of the subject

mechanisms.

CUNNECTING

FLYWHEEL

CRANKSHAFT

BELLCRANK

INTRODUCTION

Figure 1 schematically illustrates a single-cylinder piston-

displacer or "beta" type Stirling engine with a dual bellcrank

drive system. A single crankpin is linked to two ternary links or

bellcranks by ordinary connecting rods. Both bellcranks are

pivoted to the engine frame at one end. The upper bellcrank

drives the displacer via a binary link to the displacer rod. The

lower bellcrank is similarly linked to the piston.

As the crank rotates, the bellcranks oscillate and impart near

sinusoidal motion to the piston and displacer at a phase angle of

FIG. 1 A NEW TWIN BELLCRANK DRIVE MECHANISM

60" which is typical for a beta type engine. Figure 2 shows the

FOR A SINGLE-CYLINDER PISTON-DISPLACER OR BETA

sequence of motion of the engine over a complete cycle.

TYPE STIRLING ENGINE;.

1325

0-7803-3547-3-7116 $4.00 0 1996 IEEE

FIG. 2 MOTION OF THE TWIN BELLCRANK MECHANISM OFFIG. 1 OVER A COMPLETE CYCLE.

UNDERLYING PRINCIPLES

To understand how this mechanism was conceived and why it

produces an appropriate motion for a Stirling engine, consider a

simple crank and connecting rod drive. In such a drive system, as

Fig. 3 schematically indicates, the rotary motion of the crankshaft

is coupled to approximately sinusoidal reciprocating motion of a

slider acting along a line through the center of the crankshaft.

we"crank

\ Fixed

Connecting

Rod,

FIG. 3 A CRANK AND CONNECTING ROD DRIVE

PRODUCES NEAR SINUSOIDAL MOTION ALONG A LINE

THROUGH THE CENTER OF THE CRANKSHAFT.

FIG. 5 BELLCRANKS CAN BRING TWO SINUSOIDAL

MOTIONS TO ACT ALONG A COMMON LINE.

Of course the motion of a bellcrank point is oscillatory along an

arc rather than exactly linear, but the connecting rods will

accommodate this and links between the bellcranks and the piston

and displacer rod allow for the small deviation from linearity

without imposing significant side loads. If the bellcrank arms are

relatively long or the oscillation angle is relatively small, then the

motion is linear enough to see that the motion of the piston and

displacer will still be near sinusoidal. The phase separation is

180 - a because the piston and displacer motions are taken from

the bellcranks in opposing senses in this mechanism.

By adding another connecting rod to the same crankpin,

simultaneous reciprocation can be produced along a second line

as Fig. 4 depicts, and the two motions will be out of phase

according to the angle a between the lines.

FIG. 4

CAN BE TAKEN OFF A SINGLE CRANKPIN.

OTHER DUAL BELLCRANK MECHANISMS

This approach can be applied in various ways as Figs. 6 - 9

show. Each figure represents only the basic geometry of a

Stirling engine mechanism for a beta type engine. Figure 6 again

depicts the engine already discussed to illustrate how these

diagrams represent a mechanism. The diagrams show the

crankpin circle in correct size and two bellcranks in correct size

and location. The stationary pin of each bellcrank is indicated by

the largest pivot circle; the smaller circle shows the location of

the little end of the connecting rod. Connecting rods are not

1326

Substituting bellcranks for the sliders as in Fig. 5, both

reciprocating motions can be brought to act more or less along a

common line coinciding with the center axis of the piston and

displacer of the engine.

SINUSOIDAL MOTIONS DIFFERING IN PHASE

shown in these diagrams. Connecting rod length is such that the

midstroke position of the bellcrank is that shown in the diagram.

Thus the connecting rods are of equal length and oscillate more or

less equally on either side of the dotted lines which indicate the

lines along which sinusoidal oscillation is taken from the

crankpin. The centerline shown in each diagram indicates the

axis of the engine cylinder.

Although in these diagrams the bellcranks are both shown in

their mid position, they never occupy these positions

simultaneously once connccting rods are fitted. Note that one

arm of the bellcrank is perpendicular to the dotted line; the other

arm is perpendicular to the line of motion of the piston and

displacer. The angle between the dotted lines in these diagrams is

120". Since the bellcranks are reversed in sense, the phase angle

difference between the piston and displacer that results is 180 -

120 = 60". For comparison, all the mechanisms shown in figs. 6

- 9 are scaled to produce the same stroke when linked to the

piston and displacer.

Other variations are possible in which both bellcranks are

connected to the piston alnd displacer in the same sense. An

example is shown in Fig. IO. Here the phase angle between the

piston and displacer is equiil to the angle between the lines along

which motion is taken from the crank, which is 60" in the figure.

I Q

FIG.7

FIG. 10 A DIFFERENT FORM OF BELLCRANK DRIVE

FOR BETA TYPE STIRLING ENGINES.

REDUCTION TO PRACTICE

Dual bellcrank linkages can be employed in Stirling engines

intended for virtually any application, but they have special

advantages for small sizes ;and moderate power densities. This is

because dual bellcrank linkages offer compactness and low piston

side loading. A compact linkage system makes pressurized

crankcase engines practical. In most forms the bellcrank arms

that drive the piston and displacer can be made long relative to

the oscillation angle; this imposes only light side loading on the

piston and displacer even when relatively short connecting links

are used. This makes dry crankcase engines practical at moderate

power densities.

As an example of such im engine, Fig. 11 shows a complete

design for a small multipurpose beta type Stirling. The linkage

embodies the exact geomelry shown in Fig. 10. The connecting

rods are of "bent" form for clearance and certain members of the

linkage are forked or paired to present balanced loading on the

piston and displacer. Specifically, the piston bellcrank and the

displacer connecting rod arc: forked, and there are dual links to the

piston and to the displacer Iod end.

Named the "Timberline SOO" , the engine is suitable for firing

with solid wood fuel. The compact form of this mechanism

allows the crankcase to be pressurized and serve as the buffer

space. When charged with air to a mean working pressure of 2.5

atmospheres, the engine is capable of 500 Watts brake output at

800 rpm. Table 1 following gives further specifications for the

engine (V. SENFT, 1993).

FIG. 8

FIG. 9

FIGS. 6 - 9 DUAL BELLCRANK DRIVE MECHANISMS

FOR BETA TYPE STIRLING ENGINES

1327

FLYWHEEL

"IN

LINK TO PISTON

REGENERATOR

INSUL

FIREBOX

FIG. 11 THE "TIMBERLINE 500" STIRLING ENGINE

TABLE 1 TIMBERLINE 500 SPECIFICATIONS

Shaft Power: 500 W

Design Speed: 800 rpm

Working Fluid: air

Mean Pressure: 2.5 atm.

Fuel: Wood

Hot End Temperature: 600 C

Piston Diameter: 12.7 cm

Piston Stroke: 7.62 cm

Piston Swept Volume: 966 cc

Seals: Rulon J

Bearings: Sealed rolling element

Phase Angle: 60"

Internal Heater: Corrugated copper fins

External Heater: Brazed copper fins

Internal Cooler: Aluminum fins

External Cooler: Cast aluminum fins

Regenerator: Wire screen, 70% void volume

1328

VARIATIONS

In this linkage diagram, oine of the bellcranks partially hides the

other, so each bellcrank was drawn slightly offset for better

visibility. The two centerlines in the diagram are the axes of the

hot and cold cylinders. To obtain easy clearance, each bellcrank

together with its connecting links can be placed in a separate

plane which also contains the associated cylinder axis. The phase

angle depicted in this diagram is 116".

Another interesting variation of bellcrank drive for parallel

Rider type alpha engines is that shown in Fig. 15 which was

independently devised by J. David Kirk (1988). This diagram is

based on a phase angle or 90" and is scaled to yield the same

stroke as the mechanism in Fig. 14.

Many variations or alternate forms of dual bellcrank

mechanisms are possible besides those illustrated above. This

paper focused on the beta engine because it presents the most

challenging form of the "mechanism problem" for Stirling

engines (J. R. Senft, 1993). However, the same concepts can be

adapted and applied to the other types of Stirling engine. For

example, Fig. 12 shows the linkage form of Fig. 8 with a

separation angle of 105" instead of 120"

FIG. 12 A VARIATION OF THE DUAL BELLCRANK

LINKAGE OF FIG. 8.

Figure 13 shows this linkage embodied in an actual engine design

devised at Philips (Rinia & DuPre, 1946). At high operating

temperatures, this phase angle is nominally optimal for the

opposed piston alpha type engine.

FIG. 15 TWIN BELLCRANK LINKAGE FOR A RIDER TYPE

ENGINE INVENTED BY IKIRK.

All of the linkages in Figs;. 6-9 can be used for this and other

alpha type engines with the appropriate separation angle. Dual

bellcrank linkages also can be used in the gamma or split cylinder

configuration.

Non-symmetric forms of dual bellcrank linkage drives are also

possible and worthy of consideration. To illustrate, we conclude

with a linkage drive which Robert Stirling himself could have

used in his first engine. Figure 16 represents the linkage form in

the abstract and Fig. 17 shows how the very first Stirling engine

would have looked with this mechanism.

FIG. 13 THE PHILIPS "W" LINKAGE ENGINE.

For alpha type engines with side-by-side parallel cylinders, the

dual bellcrank mechanism represented by the diagram in Fig. 14

is suitable.

i i

FIG 14 MECHANISM FOR A PARALLEL CYLINDER

RIDER TYPE ALPHA ENGINE.

FIG 16 A NON-SYMMETRIC FORM OFTWIN BELLCRANK

LINKAGE FOR A BETA TYPE STIRLING ENGINE.

1 329

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: